Our long-term goal is to use a genetically tractable eukaryote, the budding yeast Saccharomyces cerevisiae, to understand septin organization, structure, and function. The septins are nucleotide-binding, filament-forming proteins encoded by a multigene family in eukaryotic organisms ranging from fungi to mammals. Septin expression, localization, and function are complex and diverse, and disruption of septin function results in pleiotropic developmental defects, indicating roles in normal development. Septins interact with each other and localize primarily to the cell cortex where they have a conserved role in cytokinesis. In S. cerevisiae,septins also function in cell-cycle progression, cell compartmentalization, bud-site selection, localized chitin deposition, mating, and sporulation. Our current working model, the "scaffold" model, suggests that septins function largely by localizing to specific subcellular regions (e.g., the mother-bud neck of S. cerevisiae) where they serve as a scaffold to recruit and maintain the localization of other proteins. The septin-directed localization of these septin-associated proteins can target them to their site of action or away from their site of action. Because of their conserved structure and function, what we learn about septins in S. cerevisiae will be valuable to our understanding of septin function in more complex, multicellular eukaryotes. Many important questions remain about septin organization, structure, and function. The proposed experiments will address a number of these questions, including: how do septins interact with each other to form multimeric septin complexes and what septin regions confer proper subcellular localization and functional diversity? Do yeast septins bind and hydrolyze a nucleotide and bind phospholipids? If so, what are the functions of these interactions? What is the role of the septin interacting protein, Bni5p, and of novel proteins in septin function?